In a blade of a bulldozer, a structure as shown in, for example, FIG. 8 is conventionally known as a blade mounting structure for inclining (hereinafter, called tilt) the blade in a left and right direction. In FIG. 8, tip end portions of a set of left and right straight frames 3 and 3 are connected to lower portions of left and right end portions of a blade 1 via connecting members J1L and J1R such as ball joints or cross joints (joints each having a pin turning in an up-and-down direction and a pin turning in a left and right direction) to be swingable up and down and to a left and a right. Base end portions of the set of left and right straight frames 3 and 3 are connected to left and right portions of a set of left and right traveling unit frames 6 at a side of a bulldozer vehicle body via ball joints J2L and J2R respectively to be swingable up an down and to the left and the right. A hydraulic cylinder 4 for tilt drive is connected to either a left or a right side (in FIG. 8, the right side facing to the front side of the vehicle) and a support member 4a is connected to the other side, between upper portions of the left and right end portions of the blade 1 and the set of left and right straight frames 3 and 3. The hydraulic cylinder 4 and the support member 4a are respectively connected with ball joints J4R and J4L on a side of the blade 1, and with pin connection J5R and J5L swingably in vertical surfaces on sides of the straight frames 3. One end portion of a link member 5 is connected to a central portion of the lower portion of the blade 1 with a pin swingably up and down. Arms 2 and 2 are connected to between the set of left and right straight frames 3 and 3 and the other end portion of the link member 5 respectively via ball joints J3C, J3L and J3R (for example, see page 2, and FIG. 1 and FIG. 2 of Japanese Utility Model No. 2546933).
As the other mounting structure example of the arms 2 and 2, there is the one provided with two arms 2a and 2a swingably mounted instead of the link 5, as shown in FIG. 9. Namely, one end portions of a set of left and right arms 2a and 2a are connected to the end portions of the set of left and right straight frames 3 and 3 on the side of the blade via ball joints J6L and J6R swingaly up and down and to the left and the right. The other end portions of the arms 2a and 2a are swingably connected to each other via a ball joint J7. The arms 2 and 2 are swingably connected to the other end portions of the set of left and right arms 2a and 2a via ball joints J8 and J8.
However, in the above-described conventional blade mounting structure, the following problems exist. The set of left and right arms 2 and 2, or the sets of left and right arms 2 and 2, and 2a and 2a are mounted to a rear side of the lower portion of the blade 1. However, a predetermined clearance (corresponding to a clearance d shown in FIG. 9) is needed to avoid interference between the set of left and right arms 2 and 2 and a front portion of the vehicle main body, and therefore the position of the blade 1 has to be located away from the vehicle body. In addition, the set of left and right arms 2 and 2, or the set of left and right arms 2 and 2, and 2a and 2a are connected to the blade 1. Consequently, it becomes difficult to secure visibility of the blade 1 from a driver's cab 7 provided at the vehicle body (especially, the visibility of a left portion of the blade from the driver's cab 7 which is normally provided to be set aside on a left side of the vehicle body), and there arises the problem that favorable workability is not provided at the time of a ground leveling operation and the like by the blade 1.
Since the number of arms 2, 2, 2a and 2a is large, the number of welded spots of the ball joints and the like which are their connecting members is large, thus causing the problem that the entire weight of the blade is increased by welding for securing the base material strength and the welding operation time is required to increase the manufacturing cost. Further, there are many spots to be supplied with grease such as ball joints, and therefore there exists the problem of reduction in maintainability.